1. Technical Field
The present invention is generally directed to an improved integrated circuit package design. More specifically, the present invention is directed to a multi-layer ceramic package in which additional shielding lines are added to reduce noise.
2. Description of Related Art
As Very Large Semiconductor Integrated (VLSI) circuits become more dense, there is a need in the art to have semiconductor packaging structures that can take full advantage of the density and speed provided by state of the art VLSI devices. Present day modules made of ceramic, typically multilayered ceramic modules, are normally mounted onto cards or boards, with cards or boards combined together to form the central processing unit (CPU) of a computer. The multilayer ceramic (MLC) modules typically have VLSI chips mounted on the top surface.
Multilayer modules are used for the packaging of electronic components, especially integrated circuit chips. Both single chip modules (SCM) and multi chip modules (MCM) are widely used. The most common type of such modules is the multilayer ceramic packaging module. In this type of module, the layers consist of a ceramic or glass-ceramic material. However, other types of thick film technologies are known, such as glass epoxy and Teflon. An example of multilayer ceramic packages is provided in U.S. Pat. No. 5,812,380, issued to Frech et al. on Sep. 22, 1998, which is hereby incorporated by reference.
As integrated circuit speeds and packaging densities increase, the importance of the packaging technology becomes increasingly significant. For example, as devices approach gigahertz speed, inductance effects in the package become more significant. Such inductance effects may arise from switching, for example, and are particularly problematic in power and ground leads. Inductance effects in the package can cause ground bounce, signal cross-talk and the like. Increasing circuit size and speed also impact the heat dissipation ability of the package.
VLSI and Ultra Large Semiconductor Integrated (ULSI) chips are especially designed for high performance applications and are thus limited by noise. The noise is caused by a high number of simultaneously switching off-chip drivers (OCD noise) and by a high number of simultaneously switching latches and the associated logic gates (logic noise). Both noise sources impact and restrict the on-chip and off-chip performance and jeopardize the signal integrity. Both noise sources generate noise due to line-to-line coupling and due to the collapse of the voltage-ground (GND) system.
A multilayer ceramic fabrication process involves the formation of the green or unfired ceramic layers or sheets, the formation of the conductive paste, the screening of the conductive paste onto the green ceramic sheets and the stacking, laminating and firing of ceramic sheets into the final multilayer ceramic structure. These general processes are known in the art and are described, for example, in U.S. Pat. No. 2,966,719 issued to Park, which is hereby incorporated by reference.
The ceramic green sheet is formed by weighing out the proper portions of the ceramic powder and glass frit, and blending the particles by ball or other milling techniques. The organic binder comprising the thermoplastic resin, plasticizer and solvents is then mixed and blended with the ceramic and glass powders on a ball mill. A slurry or slip is cast into a tape form by extruding or doctor blading. The cast sheet is then allowed to be dried of the solvent constituent in the binder system. After the tape is completely dried, it is then cut into working blanks or sheets. Registration holes are formed in the blanks together with the via holes which are selectively punched in the working blanks. The via holes will eventually be filled with a conductive composition to allow for electrical connections from layer to layer in the multilayer ceramics structure.
The wiring layers in a multi-layer ceramic package are designed in a stacked triplate configuration with the signal wiring sandwiched between upper and lower reference planes (typically alternating in vdd/gnd polarity). These reference structures are meshed in a regular grid structure to allow via interconnections for the signal and power lines. This triplate structure is a controlled impedance environment that allows high speed signal propagation.
With increased signal rising and falling edge rates and bus signaling speeds, signals on these wiring layers interact with other signals on the signal layers above and below it through the meshed reference structure. This interaction, i.e. cross-talk, between high speed signals introduces inter-symbol interference (ISI) on the nets that severely limits the maximum signaling rates and performance on these nets. ISI is the distortion of a received signal, wherein the distortion is manifested in the temporal spreading and consequent overlap of individual pulses to the degree that the receiver cannot reliably distinguish between changes of state, i.e. between individual signal elements. At a certain threshold, inter-symbol interference will compromise the integrity of the received data.
Thus, it would be beneficial to have an improved structure for multi-layer ceramic packages that reduces cross-talk between wiring layers of the package. Moreover, it would be beneficial to have an improved structure for multi-layer ceramic packages that reduces cross-talk with no loss in wiring density allowed by the technology.